1. Field of the Invention
The present invention relates to high voltage regulating circuits and, more particularly, to a high voltage regulating circuit for use in regulating a high voltage supplied to a cathode ray tube (CRT) of a television receiver or the like, for example.
2. Description of the Related Art
High voltages of from 25 kV to 28 kV must be applied to an anode of the CRT of the television receiver. Such high voltage is generally generated by a flyback transformer and must be controlled so as not to fluctuate because the high voltage fluctuation changes the size of raster or the focusing.
To regulate or stabilize the high voltage, the following two methods are known in the art:
In a first method which is known as a power source voltage modulation method, a high voltage output is detected and a high voltage is regulated by changing the power source voltage. In a second method which is known as a pulse width modulation method, a high voltage output is detected and a high voltage is regulated to be constant by changing the pulse width (pulse width of retrace).
Further, it is proposed to couple a second flyback transformer (flyback transformer for control) to the secondary side of the flyback transformer.
The above-mentioned two conventional methods are sufficient for stabilizing or regulating the high voltage. However, a tertiary output voltage of about 15 V to 200 V derived from the flyback transformer and the retrace pulse width in order to control the high voltage are fluctuated so that the television receiver must require a so-called separate system in which a horizontal deflection circuit and a high voltage generating circuit are separated and spaced apart or in substantially the same as the separate system.
Further, in the method of coupling the second flyback transformer to the secondary side of the flyback transformer, if a focusing voltage is obtained from an intermediate point of a high voltage coil, then this voltage is fluctuated due to the high voltage control. As a result, such fluctuated voltage cannot be used as the focusing voltage. Furthermore, the two flyback transformers are used, which unavoidably provides an expensive high voltage regulating circuit.
In addition, since these conventional methods are to primarily control the input voltage waveform itself of the primary winding of the flyback transformer, a large electric power portion (large current and large voltage) including the horizontal deflection circuit must be controlled, which needs the assembly parts of large capacity. From this standpoint, the conventional high voltage regulating circuit becomes expensive.